Name the world capitals that have a tropical climate as defined under the Köppen classification system.
Under the Köppen system, a location is considered to have a tropical climate if the mean temperature in the coldest month is at least 18°C (64.4°F) and it is not arid or semi-arid, regardless of latitude.
For a detailed explanation of the Köppen system, see here.
Capitals are as defined in Jetpunk's World Capitals Quiz.